Based on the beam coherence-polarization matrix and the propagation law, the polarization properties of partially coherent flat-topped light beams diffracted through a regular polygonal aperture are studied systematically. The expression for the degree of polarization of partially coherent flat-topped light beams is derived. And according to that, the expression for the degree of polarization of Gaussian Schell-model beams and the propagation of partially coherent flat-topped light beams in the free space are obtained. It is shown that the polarization properties of partially coherent flat-topped light beams diffracted through a rectangular aperture depends on the truncation parameter, spatial coherence parameters, diffraction angle, propagation distance and the order of flat-topped light beams.